Dc &amp; ac current detection circuit

ABSTRACT

A circuit for detecting a DC current in a conductor (L 1 ) includes a current transformer (CT) having a ferromagnetic core ( 10 ), a primary winding comprising the conductor (L 1 ) and at least one secondary winding (W 1 ). The circuit further includes an oscillator ( 12 ) for supplying an oscillating signal across the secondary winding and means for detecting a dc offset in the current flowing in the oscillator circuit. The circuit includes a capacitor (C 1 ) in series with the secondary winding, and the detecting means is arranged to detect a non-zero voltage across the capacitor above a certain level.

This invention relates to a circuit for detecting DC and in someembodiments AC currents.

It is often desirable to detect DC currents in circuits orinstallations. Detection of DC currents is often achieved by the use ofa shunt. Shunts have to be inserted in the circuit being monitored andthis involves direct contact with the DC supply. In many cases directcontact with the circuit being monitored is undesirable or evenimpractical. Hall Effect devices are also commonly used for detection ofDC currents, but these tend to be bulky and expensive.

Current transformers (CTs) are not normally used to detect DC currentsbecause CTs are only responsive to alternating currents and are notinherently responsive to a steady state current. However, currenttransformers have the advantage of being compact and inexpensive, andwould be an attractive means for achieving contactless detection of DCcurrents if the above technical problem could be overcome.

It is an object of the invention to provide a simple circuit using acurrent transformer to detect a DC current.

According to the present invention there is provided a circuit fordetecting a DC current in at least one conductor, the circuit includinga current transformer having a ferromagnetic core, a primary windingcomprising the conductor and at least one secondary winding, the circuitfurther including an oscillator for supplying an oscillating signalacross the secondary winding and means for detecting a dc offset in thecurrent flowing in the secondary winding.

The term “winding” is used in relation to the primary in accordance withconventional terminology, even though the primary may constitute asingle conductor passing through a current transformer core.

The detection circuit may be configured to detect AC currents as well asDC currents if the oscillator frequency is sufficiently high compared tothe frequency of the current to be detected, preferably at least anorder of magnitude higher.

The primary winding may comprise more than one conductor, in which casethe circuit will detect the vector sum of the currents flowing in theprimary conductors. In such a case the circuit may be used as a residualcurrent device.

In preferred embodiments the circuit includes at least one capacitor inseries with the secondary winding, wherein the detecting means isarranged to detect a non-zero voltage across the capacitor above acertain level, the non-zero voltage corresponding to a dc offset greaterthan a predetermined magnitude.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a basic circuit illustrating the principle of operation of theembodiments of the invention.

FIG. 2 a shows a typical hysteresis curve plotted for a ferromagneticmaterial.

FIGS. 2 b to 2 d are waveforms showing the effect of DC currents flowingin the primary winding of FIG. 1.

FIG. 3 is a circuit diagram of a first embodiment of the invention.

FIG. 4 is a circuit diagram of a second embodiment of the invention.

FIG. 5 is a circuit diagram of a third embodiment of the invention.

FIGS. 6 and 7 are schematic diagrams of an electric vehicle and atypical arrangement for charging the vehicle.

FIG. 1 shows a current transformer CT having a toroidal ferromagneticcore 10, a secondary winding W1 wound on the core, and a primary windingin the form of a single conductor L1 passing through the core aperture.The secondary winding W1 is connected to an oscillator 12, with acapacitor C1 connected in series with W1. The oscillator 12 produces analternating current H at frequency F1 which causes a current H to flowthrough W1 and C1. As the current H will flow alternately in onedirection and then in the opposite direction, the opposite directions ofcurrent flow can be represented by currents H+ and H−. When H+isincreased from zero, the core 10 will be magnetised, and thismagnetisation will increase until the core reaches magnetic saturation.If H+ is initially reduced and then reversed as represented by H−, thecore 10 will be brought out of saturation and then the magnetisation ofthe core will be reversed until the core reaches saturates again in theopposite direction. This behaviour is represented in FIG. 2 a.

The plot of FIG. 2 a is known as a B−H (hysteresis) loop, where H is thecurrent required to magnetise the core and B is the magnetic fluxproduced by current H. It can be seen that current H+ starts from zeroat point 0 and is increased with a positive polarity until magneticsaturation occurs at point a (positive saturation), at which point H+ isreduced and then reversed to become H− to take the hysteresis loop frompoint a through point b through point c to negative saturation point d.The current is again reduced and then reversed to become H+ which takesthe loop from point d through point e through point f and back to pointa again.

This process will continue in an oscillatory manner as determined byfrequency F1 which, in order to detect AC as will be described, willnormally be substantially higher than normal mains supply frequency of50 or 60 Hz, e.g. at least an order of magnitude higher and preferablyat least 1500 Hz and most preferably around 3 KHz. It can be seen thatafter the initial saturation point is reached, the core will have aresidual magnetism when H is zero, as shown at points b and e.Magnetisation will be zero when current H has a positive or negativevalue as shown at points c and f.

FIG. 2 b shows the corresponding waveform for the current flow H in theoscillator circuit. It can be seen that the current reaches a peak ineach polarity during each cycle of the oscillator frequency.

Under normal conditions (no current flowing in L1), the AC current Hflowing in the oscillator circuit will have a mean DC value of zero. Ifa DC current +I_(dc) is passed in a certain direction through theconductor L1 in FIG. 1, the current flowing in the oscillator circuitwill be shifted from the mean zero level to a mean positive level asindicated by FIG. 2 c, the magnitude of the DC offset being proportionalto the DC current flow in L1. Conversely, if the DC current flow in L1is reversed to become −I_(dc), the DC offset will also be reversed, asshown in FIG. 2 d. The DC offset that occurs in the oscillator circuitcan be used to detect and measure a DC current flow in the primaryconductor L1, as shown in the embodiment of FIG. 3.

In FIG. 3 the winding W1 is separated into two secondary windings W1 a,W1 b wound separately on the core 10, and the capacitor C1 is connectedin series between the secondary windings (in FIGS. 3 and 4 the toroidalcore 10 is shown schematically). As in FIG. 1 the primary conductor L1passes through the core aperture. The oscillator 12 is supplied with a15V supply from Vcc to ground. The oscillator 12, the two windings W1 a,W1 b and the capacitor C1 form a loop or a first circuit for currentflow from Vcc to ground. The oscillator current as represented by H+ andH− will flow back and forth through W1 a, C1 and W1 b at the oscillatorfrequency F which, as mentioned, will typically be substantially higherthan the normal mains supply frequency of 50 Hz, for example about 3KHz.

During the positive half cycles Vcc will be distributed approximately as15V, 7.5V, 7.5V and 0V at points 1, 2, 3 and 4 respectively, and duringthe negative half cycles Vcc will be distributed approximately as 0V,7.5V, 7.5V and 15V at points 1, 2, 3 and 4 respectively. So, whilstpoints 1 and 4 will swing fully between 15V and ground, the voltages atpoints 2 and 3 will remain relatively stable at 7.5V. In the absence ofany current flow in the primary conductor L1 the DC current through thesecondary windings and the DC voltage across C1 will be substantiallyzero. When a DC current +_(Idc) or −_(Idc) flows in the primary circuita resultant DC shift as shown in FIG. 2 c or 2 d will occur and thecurrent in the oscillator circuit will now have a DC offset with theresult that the voltages at points 2 and 3 will no longer be the same.The resultant differential voltage at those points can be used tocontrol rectifying means (bipolar transistors in the present embodiment)and so detect the DC current in the primary conductor L1.

To this end, a second circuit to ground is formed by bipolar transistorsTr1 and Tr2, a resistor R1 and a further capacitor C2. When a DC current_(Idc) flows in the primary conductor L1 the difference in the DCvoltage between points 2 and 3 will increase proportionately. When_(Idc) is of a certain polarity and of sufficient magnitude,corresponding to a DC offset greater than a pre-determined magnitude,point 2 will reach approximately 0.7V higher than point 3 during theoscillator cycles and transistor TR2 will start to conduct. This willallow the DC current to flow to ground via resistor R1 and develop avoltage across R1 and capacitor C2 will charge up to a certain voltage.When _(Idc) is of the same magnitude but of opposite polarity point 3will reach approximately 0.7V higher than point 2 during the oscillatorcycles and transistor TR1 will conduct. The oscillator current willagain flow to ground via resistor R1 and develop a voltage across C2.Thus a DC voltage will be developed across C2 which will be proportionalto the DC current flowing in L1 and, therefore, to a predeterminedmagnitude of DC offset in the oscillator circuit.

In the arrangement of FIG. 3 the transistors Tr1 and Tr2 are used to“siphon off” the DC component in the oscillator current by way of arectifying action so as to detect a DC current flow in the primaryconductor, the capacitor C1 allowing the high frequency current producedby the oscillator 12 to bypass the transistors. To this end, Tr1 and Tr2are used as current control means to control the current flow into C2.

If there is a DC current in L1, the voltage across C1 will always reacha level where Tr1 starts conducting independent of the magnitude of thecurrent and the value of C1. The small capacitor C1 is intended toabsorb the AC ripple current, typically at 3 KHz, produced by theoscillator 12. The ripple voltage across C1 stays well below 0.7V. Thevalue of the capacitor C1 is much smaller than that of the capacitor C2and its part in time delay is negligible. The tripping threshold isadjusted by suitable choice of the values on the components R1, C2 andthe number of turns in the windings W1 a and W1 b.

FIG. 4 shows a second embodiment of the invention wherein a singlesecondary winding W1 is used but the rectifying means is split and thesingle capacitor C1 has been replaced by two capacitors C1 a and C1 band the secondary winding W1 is connected in series between thecapacitors. The oscillator currents H+ and H− now flow through W1 andcapacitors C1 a and C1 b. Under normal conditions (no current flowing inL1) during the oscillator cycles Vcc will be distributed approximatelyas 15V, 15V, 0V and 0V at points 1, 2, 3 and 4 respectively duringpositive half cycles, and during the negative half cycles Vcc will bedistributed approximately as 0V, 0V, 15V and 15V at points 1, 2, 3 and 4respectively, and the effective differential voltage across each of C1 aand C1 b will be zero.

When a DC current of a certain polarity flows in L1 a resultant DC shiftas shown in FIG. 2 c will occur and capacitors C1 a and C1 b willacquire a charge. When the differential voltage across each of themreaches about 0.7V a diode D1 and a bipolar transistor Tr2 will conductand the DC component in the oscillator current will flow from point 1through D1, through W1 and into the emitter of Tr2, and the resultantcollector current of Tr2 will flow into R1 and cause a DC voltage to bedeveloped across C2.

When a DC current of the opposite polarity flows in L1 a resultant DCshift as shown in FIG. 2 d will occur and capacitors C1 a and C1 b willacquire a charge. When the differential voltage across each of themreaches about 0.7V a diode D2 and a bipolar transistor Tr1 will conductand oscillator current will flow from point 4 through D2, through W1 andinto the emitter of Tr1, and the resultant collector current of Tr1 willflow into R1 and cause a DC voltage to be developed across C2.

The main advantage of the arrangement of FIG. 4 is that the CT can nowcomprise a single winding rather than two windings. However, thisadvantage is offset to some extent by the fact that it requires a higherconducting threshold of 1.4 V over two capacitors in FIG. 4 comparedwith 0.7 V over one capacitor in FIG. 3 to achieve the same performance.

The voltage across R1 is smoothed by C2, and the DC voltage Voutdeveloped across R1 will be proportional to the DC current flow in theprimary conductor L1. This voltage can be monitored and used formeasurement purposes or for detection purposes such as RCD (residualcurrent device) applications where, for example, the DC current flowingin the primary conductor L1 is the difference in the currents in theline and neutral conductors of a mains electricity supply. For example,an electronic circuit 14 comprising an integrated circuit type WA050 maybe connected across the resistor R1. The WA050 is an industry standardRCD IC supplied by Western Automation Research & Development Ltd,Ireland.

The circuit arrangements of FIGS. 3 and 4 can also be used to detect ACcurrent flow in the primary conductor L1 at a typical mains supplyfrequency of 50 Hz. This is because the frequency of the oscillator 12is sufficiently high compared to the mains frequency, e.g. 3 KHz ascompared to 50 Hz, that each half cycle of primary current will beeffectively a DC current for the half cycle period, and during thisperiod the oscillator 12 will have completed multiple cycles duringwhich oscillator current will flow through R1. The DC offset in theoscillator current will therefore vary relatively slowly during eachhalf cycle of the primary current. It will be appreciated therefore thatcircuits according to the invention can be employed to detect ACresidual currents or composite AC residual currents with frequenciesfrom 1 Hz up to and approaching any selected oscillator frequency.

By suitable selection of component W1 and C1, the circuit can beoptimised to detect DC or AC currents of selected magnitudes flowing inthe primary conductor L1.

The circuit can be designed to produce the same DC offset, andaccordingly the same level of Vout, for AC and DC currents ofapproximately the same RMS value, i.e. to have in effect the sameresponse to AC and DC currents, or the circuit can be made lessresponsive to AC currents without changing the DC detection threshold ifdiscrimination between AC and DC currents is required. This can be doneby changing the value of C1 in FIG. 3. For low values of capacitance, ACcurrents will mainly flow into the emitters of the rectifiertransistors. By increasing the value of C1 the circuit will become lessresponsive to AC currents because an increasing part of the current isneeded to charge capacitor C1 to a voltage level Vbe of 0.7 V abovewhich the bipolar transistors start conducting and it will take an ACcurrent of larger magnitude to cause the same level of Vout incomparison to a given DC current.

By suitable selection of the value of capacitor C1, a threshold ofdetection for AC currents can be set independently of the DC currentlevel and in this way the circuit can be made responsive to DC currentsof a certain magnitude whilst being effectively blind to AC currents ofa similar or higher magnitude and thereby provide a very high level ofdiscrimination between AC and DC currents.

Similarly, by suitable selection of the values of capacitors C1 a and C1b in FIG. 4, threshold of detection for AC currents can be setindependently of the DC current level.

It may be desirable to set the AC current detection thresholdindependent of and at a lower value than a corresponding DC current.FIG. 5 shows an embodiment which achieves this.

In FIG. 5, components R3, R5 and C3 (the left chain) have been insertedbetween C1 and Tr1, and components R4, R6 and C4 (the right chain) havebeen inserted between C1 and Tr2. In the case of a DC current flow abovea certain level in the primary circuit (Li) the offset current producedby C1 will be DC, and a resultant current will flow through the leftchain for one polarity of the primary DC current, and through the rightchain for a DC current of the opposite polarity. Capacitors C3 and C4will act as DC blocks, so all of the offset current will be forced toflow via R5 or R6 as applicable. In the case of an AC current flow abovea certain level in the primary circuit, the resultant

DC offset current produced by C1 will have an AC component, e.g. 50 Hz.For one half cycle of the AC primary current the resultant offsetcurrent will flow via the left chain, and it will flow through the rightchain for the other half cycle. However, in the case of the ACcondition, capacitors C3 and C4 will provide an additional path forcurrent flow and thus for an AC current there will be less impedancebetween C1 and the Tr1 or Tr2 with the result that the charge on C2 willbe greater for a given AC current than it would be for the correspondingDC current. Thus the AC current threshold can be set lower than the DCcurrent threshold.

FIG. 6 is a schematic diagram of an electric vehicle (EV) 100 and atypical arrangement for charging the vehicle.

The EV 100 comprises a body 102 shown in dashed lines having anexternally accessible mains connecter 104 through which the EV can becharged from an AC mains socket 106 via a three core cable 108comprising live L, neutral N and protective earth PE conductors. Themains socket 106 is located in a building, outhouse, garage or otherfixed location, not shown.

The EV 100 converts the AC mains supply to DC to charge a battery 110using an inverter 112. The operation of inverters is well known to thosefamiliar with the art, but basically the inverter chops up the 50 Hz ACmains current at a substantially higher frequency rate (e.g. severalKHz) such that each half cycle of the mains supply becomes a highfrequency signal within respective positive and negative going halfcycles. This gives rise to a multifrequency or high frequency currentwith a DC component. When it is desired to operate the EV 100 the DCoutput from the battery 110 is converted back to AC using a secondinverter 116 to drive a motor 114.

Generally the DC supply is isolated from the protective earth PE and theEV body 102 to minimize the risk of electric shock in the event of aperson touching one side of the DC supply within the EV. An insulationmonitoring device (IMD) 118 is normally fitted within the EV to detectan inadvertent connection from the DC supply conductors to the PE, forexample due to insulation breakdown. In such a case an audible orvisible alarm will be activated. However, the IMD 118 is not aprotective device and generally does not prevent the continued use ofthe EV after an insulation breakdown has occurred. Thus, after a firstfault within the EV a shock risk can arise and the user may be exposedto such risk.

In FIG. 6 the EV 100 is connected to a standard TN system single phase230V/50 Hz AC supply comprising of live, neutral and protective earthconductors. Once the EV is so connected, its body will be at earthpotential and shock risks will be minimized. The DC supply within the EVis normally isolated from the EV body. The installation supplying the EVis normally protected by a conventional RCD, shown schematically at 120,which is based on IEC61008 or a GFCI to UL943 with a normal trip currentlevel not exceeding 30 mA.

An RCD based on IEC61008 is required to detect an AC residual current upto its rated trip level, e.g. 30 mA, at rated frequency, e.g. 50 Hz or60 Hz. Such RCDs are generally blind or non responsive to DC residualcurrents or currents at substantially higher frequencies than the normalmains supply. Furthermore, the operation of such RCDs may be impaired bythe presence of DC residual currents or residual currents at highfrequencies or at composite frequencies. Such limitations in theoperation of conventional RCDs are accepted because residual currents atDC or at high frequencies or multifrequency residual currents rarelyoccur in domestic installations. However, the emergence of the EV in themass market has given rise to a new and potentially hazardous problem.

For example, an insulation breakdown could occur between the DC supplyand the earthing system or body of the EV. This condition could occurduring normal everyday use of the EV or could occur during the chargingprocess. The IMD 118 will be activated in the event of such a fault.However, if the fault condition is ignored for any reason and the EV isconnected to the AC supply a current _(Idc) will flow from the DCconductor through the PE to the supply live and through the RCD 120 as adifferential current back to the DC conductor. This current will have DCcomponents and will be at a relatively high frequency as determined byinverter 112. Given that the voltage at the output of inverter 112 canbe in the range of several hundred volts, the resultant residual currentwill be relatively high and in any event substantially higher than the30 mA rating of the conventional RCD. The effect of _(Idc) will be todesensitize the RCD 120 to such an extent that its normal 50 Hz triplevel will be increased well above its rated trip level and therebyimpair its ability to provide adequate shock protection against asubsequent 50 Hz residual current.

A second fault condition could occur where a person touches a live partof the AC supply. The resultant residual current I_(ac) will also flowthrough the RCD 120 but is highly likely to go undetected due to thepresence of the first fault current _(Idc). Thus, because of a faultwithin the EV, the normal protection provided by the RCD 120 has beencompromised. A similar problem would arise in the case of a breakdown ofthe insulation between the output of the inverter 116 and the protectiveearth. The potential hazard is not confined to the immediate vicinity ofthe charging circuit. The

EV may be connected to a charging circuit outside a house. The house andall socket outlets are protected by a conventional RCD, as is standardpractice. When the EV with the insulation breakdown problem is connectedto a socket outlet in a garage or car port, the RCD protecting theentire installation will be compromised, and a person touching a livepart within the house during the EV charging operation may no longerhave the expected shock protection previously afforded by the RCD.

Although an alarm may be activated by the IMD 118 for the first faultwithin the EV, the user would have no way of realizing that this faultcould cause failure of an RCD 120 in an external installation when theEV is connected to that installation. Thus, a shock hazard could begenerated within an external installation due to a fault within the EVand conventional solutions would not provide adequate protection. The EVcould be left connected overnight or even over a weekend, in which casea sustained shock hazard would arise.

This problem could be overcome by replacing the conventional RCD 120with a B Type RCD based on IEC62423 which is designed to detect DCresidual currents and AC residual currents up to 1 KHz. However, suchRCDs are substantially more expensive than conventional RCDs and aretherefore unlikely to be used in residential applications. Furthermore,it would not be feasible to replace millions of conventional RCDsworldwide just to mitigate this problem.

The insulation breakdown as described will generally go undetectedunless special equipment such as an IMD is fitted within the EV todetect it. Failure of the IMD itself could cause a resultant breakdownin insulation between the DC supply and the PE and possibly cause thevery problem that compromises the external RCD. There is no way ofensuring that the user will not connect the EV to an external supply tocharge the battery under a fault condition, especially if the userbelieves the fault and the resultant risks to be contained within theEV, or that there is simply a false alarm.

FIG. 7 shows a simple but effective means to mitigate the above problem.

In FIG. 7 a residual current device (RCD) 122 has been fitted within theEV to monitor the current flowing in the PE conductor within the EV. Inthis arrangement, the RCD 122 is constructed as described in any of thepreceding embodiments and uses a current transformer CT having atoroidal core 10 which surrounds the live L and neutral N supplyconductors to detect a current imbalance in those conductors indicatinga residual current flowing in the PE conductor. In the event that aresidual current in excess of a predetermined threshold flows in the PE,the RCD IC (WA050) 14 will produce an output. This output can be used todisconnect the EV from the external supply by opening contacts SW1 inthe AC live and neutral conductors, and also advantageously the PEconductor. The shock hazard will then be removed regardless of theactions of the user. (It will be appreciated that the RCD 122 is shownin highly simplified form in FIG. 7, and shows only the toroidal core 10and the IC 14, all intermediate components being omitted). The RCD 122is designed to detect residual currents over the range of DC to thehighest frequency component produced by the inverter 112. The CT core 10may encompass the active L and N conductors together as shown in FIG. 7and detect DC or AC residual current flow in the AC supply to the EV. Analternative option would be for the CT core 10 to encompass the PEconductor alone, but if the body of the EV was externally grounded, theresidual current could be split between the PE and the external returnpath and possibly go undetected.

The invention is not limited to the embodiments described herein whichmay be modified or varied without departing from the scope of theinvention.

1. A circuit for detecting a DC current in at least one conductor, thecircuit including: a current transformer having a ferromagnetic core, aprimary winding comprising the conductor and at least one secondarywinding, an impedance in series with the secondary winding, anoscillator for supplying an oscillating signal across the impedance andthe secondary winding and means for detecting a dc offset in the currentflowing in the impedance and the secondary winding, wherein: theimpedance comprises a first capacitor, the current transformer has twosecondary windings and the first capacitor is connected in seriesbetween the secondary windings, and the detecting means is arranged todetect a non-zero voltage across the first capacitor above a certainlevel, the non-zero voltage corresponding to a dc offset greater than apredetermined magnitude, and wherein the detecting means comprises: apair of transistors each of which is turned on in response to a detectednon-zero voltage in a respective one of the two opposite directionsacross the first capacitor, a resistance and a second capacitor mutuallyconnected in parallel, the parallel combination of the resistance andsecond capacitor being connected in series with each transistor so thatthe second capacitor is charged up when either transistor is turned on,and means for monitoring the voltage on the second capacitor. 2-10.(canceled)
 11. A circuit as claimed in claim 1, wherein the oscillatorfrequency is at least an order of magnitude higher than mains supplyfrequency to allow the detection of mains frequency AC currents in theconductor.
 12. A circuit as claimed in claim 11, wherein the value of atleast one circuit component is selected so that the same DC offset isobtained in respect of AC and DC currents having different RMS values.13. A circuit as claimed in claim 11, wherein the value of at least onecircuit component is selected so that the DC offset obtained in respectof DC currents is substantially greater than that for AC currents ofequivalent RMS value.
 14. A circuit as claimed in claim 11, wherein thevalue of at least one circuit component is selected so that the DCoffset obtained in respect of DC currents is substantially less thanthat for AC currents of equivalent RMS value.
 15. A circuit as claimedin claim 1, wherein the current transformer comprises an apertured core,the conductor passing through the aperture and the, or each, secondarybeing a winding on the core.
 16. A circuit for detecting a DC current inat least one conductor, the circuit including: a current transformerhaving a ferromagnetic core, a primary winding comprising the conductor,and a secondary winding, at least one impedance in series with thesecondary winding, an oscillator for supplying an oscillating signalacross the impedance and the secondary winding, and means for detectinga dc offset in the current flowing in the impedance and the secondarywinding, wherein: the at least one impedance comprises a first andsecond capacitors, the secondary winding is connected in series betweenthe first and second capacitors, and the detecting means is arranged todetect a non-zero voltage across each of the first and second capacitorsabove a certain level, the non-zero voltage corresponding to a dc offsetgreater than a predetermined magnitude, wherein the detecting meanscomprises: a pair of transistors each of which is turned on in responseto a detected non-zero voltage in a respective one of the two oppositedirections across the secondary winding, a resistance and a thirdcapacitor mutually connected in parallel, the parallel combination ofthe resistance and third capacitor being connected in series with eachtransistor so that the third capacitor is charged up when eithertransistor is turned on, and means for monitoring the voltage on thethird capacitor.